destutil: remove duplicate check and leave it to merge.update()
The check is done in merge.update() already and the next few patches
will add more checks there. Some of the additional checks will need
information about the merge that will not be available in destutil.
Since commands.postincoming() catches UpdateAbort(), we need to change
merge.update() to raise that more specific exception.
This goes directly again
45b86dbabbda (destupdate: move the check
related to the "clean" logic in the function, 2015-10-05), but it will
simplify the next few patches, and we can always move it out again
(preferably move, not copy) after if we still think it's better that
way.